The present invention relates to a pressure sensitive adhesive composition based on an acrylic copolymer which is at least in part an acetoacetyl group-containing acrylic copolymer.
Pressure sensitive adhesives have heretofore been used in various fields of application, in such varied forms as the adhesive tape, two-sided adhesive tape, adhesive label or sticker, adhesive sheet and so on.
Particularly, pressure sensitive adhesives of acrylic copolvmer type have been used in an extended spectrum of application by virtue of their unexcelled weather resistance.
Among the acrylic copolymers in common use for pressure sensitive adhesives are those copolymers which can be produced by the copolymerization of, as a major component unit, a monomer having a low glass transition temperature such as an alkyl acrylate having about 2 to 12 carbon atoms in the alkyl moiety or an alkyl methacrylate having about 4 to 12 carbon atoms in the alkyl moiety and, as a minor component unit, a monomer having a comparatively high glass transition point such as methyl acrylate, an alkyl methacrylate having 1 to 3 carbon atoms in the alkyl moiety, vinyl acetate, acrylonitrile, methacrylonitrile, styrene or the like, and/or a functional group-containing monomer such as acrylic acid, methacrylic acid, itaconic acid, maleic anhydride, a hydroxyalkyl methacrylate, dimethylaminoethyl methacrylate, acrylamide, methacrylamide, methylolacrylamide, glycidyl methacrylate and so on. Regarding said functional group-containing monomer, it is generally acknowledged that when acrylic acid is used in the copolymerization reaction, treatment of the copolymer with a small amount of a crosslinking agent such as an inocyanate compound, a metal chelate compound, an amino resin or an epoxy resin results in improvements in cohesive strength and heat resistance.
The specification of Japanese Patent Publication No. 50166/1984, my copending application, describes a resin composition comprising a copolymer containing, at least in part, an acetoacetyl group-containing ethylenically unsaturated monomer unit and an amino resin. This literature is mainly directed to a coating composition but mentions that the same composition can be used as an adhesive (pressure sensitive adhesive) as well (Example 14).
However, the acrylic copolymer-based pressure sensitive adhesives heretofore in common use are lacking in the balance of three cardinal qualities of adhesivity, namely tack, adhesion and cohesion, and have room for improvement. On the other hand, when a carboxyl group-containing monomer such as acrylic acid is used in the copolymerization reaction, the resulting resin composition tends to cause corrosion of metal, if such is the adherend.
The pressure sensitive adhesive disclosed in Japanese Patent Publication No. 50166/1984 has generally the disadvantage of a short pot life when an amino resin is added and provides only an insufficient keying or anchor strength when the substrate is a polyester film.
It is a primary object of the present invention to provide a pressure sensitive adhesive composition which is not only well-balanced in the aforementioned triad of adhesivity but has a sufficient keying strength and is conducive to a long pot life after addition of a curing agent.